In the whole semiconductor fabrication process, an ion implantation is a physical step in which impurity ions are implanted into a wafer in order to provide the wafer with modified electrical characteristics from its natural state. In other words, the technique is used to select and accelerate impurity ions of a certain species and of a certain quantity so as to implant the impurity ions into a particular portion of the wafer to a needed depth. As compared to a thermal diffusion, the ion implantation can markedly reduce diffusion of the impurity ions into sides of an impurity region. In addition, it is possible to undergo the process at a lower temperature than if diffusion were used, so that the impurity region can be minutely formed without degrading a photoresist. Accordingly, while overcoming disadvantages of the thermal diffusion, the ion implantation has been extensively used for the semiconductor fabrication processes.
Each of ion implanting systems is configured with an ion generator; a beam line; and an end station, and their construction diagrams are generally almost the same. The detailed description of this is found in U.S. Pat. No. 4,672,210. According to process conditions, the ion implanting systems can typically be categorized as mid-current ion implanting systems, high-current ion implanting systems, or high-energy ion implanting systems. Their constructions may be a little different depending on manufacturers and models.
The ion implanting system enables selecting and accelerating of ions of a needed quantity required for forming layers, thereby implanting the ions into the wafer. At this time, the wafer is positioned on a wafer holding apparatus.
FIG. 8 illustrates problems that may arise when the wafer is mounted on an end station of a conventional ion implanting system.
A reference numeral “w” denotes a wafer where semiconductor elements are fabricated; a reference numeral “10” denotes one sidewall of an ion implanting chamber; a reference numeral “20” denotes an electro static chuck (ESC); and reference numerals “30” and “40” denote an x-axis rotating part for controlling a tilt angle of the ESC with respect to an x-axis and a y-axis rotating part for controlling a tilt angle of the ESC with respect to a y-axis, respectively. For example, the tilt angle of the wafer w with respect to an ion beam is preferably adjusted to be about 7 degree when the ion beam is projected against the wafer.
The tilt angle of the ESC 20 illustrated in FIG. 8 is determined by encoding movements of motors when the motors (not shown) included in the x- and y-axis rotating parts 30 and 40 are driven on the basis of a hard stop. According to the determined tilt angle, the position and the tilt angle of the ESC 20 are set. In case the position of the hard stop is swerved due to errors of the apparatus, the initial position of the ESC 20 may be mistakenly set. Thus, in the ion implantation process, the ESC 20 tilted toward one side may be recognized as an initialized condition, thus disabling ion implantation into a predetermined position of the wafer 10. Consequently, the semiconductor devices cannot obtain characteristics as required.
As mentioned above, the conventional wafer holding apparatus has been dependant upon a mechanical alignment without any sensing system for maintaining and managing the angle of the ESC 20. Thus, it was impossible to sense and compensate positional deviation that could arise after employing the apparatus for a long time.
Besides, the conventional wafer holding apparatus for the ion implanting system reads an encode value of the motor and feedbacks the tilt angle to an operation interface so as to compensate the tilt angle. However, since the conventional apparatus reads the encode value of the motor as a tilt angle of a surface of the ESC 20, the substantial tilt angle of the ion beam with respect to a surface of the wafer can be changed not only by a mechanical structure, for example, a connection structure like an axis of rotation, but also by deviation of the incidence angle of the ion beam. Therefore, it cannot be seen that the conventional wafer holding apparatus substantially feedbacks the tilt angle to the operation interface.